1. Field of the Invention
The present invention relates to a silica-based optical fiber that is used for optical fiber communication, and more particularly, to a method of measuring a minimum wavelength of a single-mode fiber for which the propagation mode is single, that is, a cut-off wavelength in the fiber.
2. Background Art
As a conventional method of measuring the cut-off wavelength of a single mode fiber, particularly, as a method of measuring an effective cut-off wavelength, for example, as illustrated in “Opto-electronic Measuring Instruments Guide, Thoroughly-Revised Edition”, Optronics Co., Ltd., Published on Jun. 24, 2004, a bending method and a multi-mode excitation method are known.
In the bending method, a cut-off wavelength is calculated by using the difference between loss properties measured in a state in which a bending portion is not added to a single-mode fiber and a state in which a bending portion is added to the single mode fiber.
Schematic configurations of measurement systems for measuring a fiber cut-off wavelength according to the bending method by using a strand having a length of 2 m are illustrated in FIGS. 11A and 11B.
FIG. 11A illustrates a case where a bending portion is not effectively added to a measurement target fiber.
FIG. 11B illustrates a case where a bending portion is effectively added to the measurement target fiber.
In FIGS. 11A and 11B, reference numeral 10 is a light source unit.
The light source unit 10, for example, is configured of a white light source and a spectroscope that spectrally disperses white light emitted from the white light source.
The light emitted from the light source unit 10 is guided by a measurement target fiber 12 so as to be received by a light receiving unit 14.
As the measurement target fiber 12, the same single mode fiber is used in the case where a bending portion is added thereto (FIG. 11B) and a case where a bending portion is not added thereto (FIG. 11A).
In a case where measurement is performed without a bending portion being added to the measurement target fiber 12 (FIG. 11A), transmitted light is received by the light receiving unit 14 in a state in which a bending portion is not effectively added to the measurement target fiber 12.
On the other hand, in a case where measurement is performed with a bending portion being added to the measurement target fiber 12 (FIG. 11B), transmitted light is received by the light receiving unit 14 in a state in which a small bending portion B1 (for example, a bending portion of 60 mmφ is added to the measurement target fiber 12.
In the states illustrated in FIGS. 11A and 11B, the wavelength dependence of the transmission light power is measured.
A cut-off wavelength is calculated based on the ratio between the transmission light power in a case where there is bending and the transmission light power in a case where there is no bending.
Accordingly, the above-described bending method may be called a measurement method using a difference in bending loss between a base mode and a higher order mode of a single mode fiber.
In the cases illustrated in FIGS. 11A and 11B, a bending portion B2 of 280 mmφ is disposed on the measurement target fiber 12.
This is a bending portion that is determined based on standards.
The bending portion of 280 mmφ is a bending portion that does not substantially cause any bending loss.
In the description presented here, the “standards” represent standards on the measurement of a cut-off wavelength such as IEC 60793-1-44 “Measurement methods and test procedures—Cut-off wavelength”, ITU-T G. 650.1 “Definitions and test methods for linear, deterministic attributes of single-mode fibre and cable”, and JIS C 6825 “Test methods for structural parameters of optical fibers—Optical characteristics”.
On the other hand, the multi-mode excitation method is a method of measuring a cut-off wavelength based on the ratio between the power of transmitted light transmitted through a multi-mode fiber (reference fiber) used as a reference, for example, a multi-mode fiber having a short length of 1 to 2 m and the transmission light power in a case where the measurement target fiber is excited in a multi-mode.
Schematic configurations of measurement systems used when a cut-off wavelength is measured by using the multi-mode excitation method are illustrated in FIGS. 12A and 12B.
FIG. 12A illustrates a measurement method using a reference fiber.
As the reference fiber 16, a multi-mode fiber having a short length is used, and the transmission light power of the reference fiber 16 is measured.
On the other hand, FIG. 12B illustrates a measurement method using a measurement target fiber.
Here, a reference fiber 16 is prepared which is configured of the same multi-mode fiber as the fiber used in the reference measurement.
The measurement target fiber 12 is connected to the output end of the reference fiber 16, and light transmitted through the reference fiber 16 and the measurement target fiber 12 is received by the light receiving unit 14 so as to measure the power of the transmitted light.
The cut-off wavelength of the measurement target fiber 12 is calculated based on the wavelength dependence that is represented by the ratio between the power of transmission light transmitted through the reference fiber 16 and the power of transmission light transmitted through the reference fiber 16 and the measurement target fiber 12.
This multi-mode excitation method is a method using a phenomenon in which the power of transmission light transmitted through the measurement target fiber 12 markedly changes in a wavelength region in which the mode is switched from a multi-mode to a single mode.
For measuring the measurement target fiber using the multi-mode excitation method (FIG. 12B), two bending portions (B3 and B4) of 80 mmφ formed so as to make one complete rotation around the measurement target fiber 12 are disposed at both ends of the measurement target fiber.
The reason for disposing the bending portions is that it is defined in the standard that one circulation of a bending portion of 80 mmφ and a plurality of circulations of a bending portion of 280 mmφ are arranged at both ends of a sample in a case where the cable cut-off wavelength is measured by using the sample having a length of 22 m.
In addition, in a case where the cut-off wavelength of a fiber using a strand of 2 m as the length of the measurement target fiber 12 is measured, similarly to the above-described bending method, the bending portions B3 and B4 are not disposed.
As cut-off wavelengths defined in the standards, there are a cable cut-off wavelength measured by using a fiber cable having a length of 22 m, a cable cut-off wavelength measured by using a fiber strand having a length of 22 m, a fiber cut-off wavelength measured by using a fiber strand having a length of 2 m, and a jumper cable cut-off wavelength measured by using a jumper cable having a length of 2 m.
In the methods of measuring the cut-off wavelength, while the methods of measuring the measurement target fiber are different from each other, as the method of measuring the reference fiber, one of the bending method and the multi-mode excitation method is used.
Incidentally, recently, low-bending loss optical fibers such as an HAF (Hole Assisted Fiber) and a trench-type optical fiber have been actively developed.
The low-bending loss optical fibers of such a kind are designed such that the effect of confining light in the core is increased.
In such low-bending loss optical fibers, the confinement effect is also increased for a higher order mode, and accordingly, it is difficult to remove the higher order mode with the degree of bending that is applied in the bending method.
Accordingly, it is difficult to calculate the cut-off wavelength with accuracy in the bending method.
Therefore, conventionally, in order to measure the cut-off wavelength of the low-bending loss optical fiber of such a kind, the multi-mode excitation method is generally used.
Now, the method of measuring the cut-off wavelength according to the multi-mode excitation method will be described in detail with reference to FIG. 13.
First, as described above, the power Psig(λ) of transmission light transmitted through a measurement target fiber and the power Pref(λ) of transmission light transmitted through a multi-mode fiber used as a reference fiber are measured, and the ratio A(λ) of the transmission light power Psig(λ) to the transmission light power Pref(λ) is acquired as a logarithmic ratio using A(λ)=10×log10 {Psig(λ)/Pref(λ)}.
The wavelength dependence spectrum of the ratio A(λ) is denoted by a thick solid line 18 in FIG. 13.
Here, in the measurement target fiber, from a long wavelength side toward a short wavelength side, the power Psig(λ) of the transmission light transmitted through the measurement target fiber drastically increases at a position close to a wavelength changing from the single mode propagation region (a region in which only light of the base mode is propagated) to the multi-mode propagation region (a region in which not only light of the base mode but also light of the higher order mode is propagated).
Accordingly, the wavelength dependence of the ratio A(λ) of the power Psig(λ) of transmission light transmitted through the measurement target fiber to the power Pref(λ) of transmission light transmitted through the multi-mode fiber used as a reference fiber, as illustrated in FIG. 13, also drastically increases from the long wavelength side to the short wavelength side at a position close to the wavelength for which the measurement target fiber changes from the single mode propagation region 20 to the multi-mode propagation region 22.
Thus, a straight line that is acquired by linearly approximating the spectrum, which is positioned on the long wavelength side corresponding to the single mode propagation region, of the measurement target fiber in compliance with a standard such as JIS will be referred to as a reference line (a thin solid line 24 shown in FIG. 13).
In addition, a wavelength corresponding to an intersection 28 of a straight line (a dashed-two dotted line 26 shown in FIG. 13) acquired by shifting the reference line by 0.1 dB in a parallel manner and the spectrum of the transmission light power ratio A(λ) is defined as a cut-off wavelength λ.
However, in the cut-off wavelength measurement according to the multi-mode excitation method, there are the following problems.
Since multi-mode propagation is performed by using the multi-mode fiber as the reference fiber, a variation (a variation in the shape of a wave shape, a ripple shape, or a hump shape) occurs over the entire multi-mode propagation wavelength region in the power Pref(λ) of the transmission light due to the wavelength dependence of the loss property.
For the power Psig(λ) of the transmission light transmitted through the single mode fiber used as the measurement target fiber, in a region (in other words, the single mode propagation region in which only light of the base mode is propagated) 20 located on the long wavelength side of the cut-off wavelength, single mode propagation is performed.
Accordingly, a large variation in the wave or the like does not substantially occur.
In addition, also in the transmission light power ratio A(λ), due to the wave occurring in the wavelength dependence of the loss property of the reference fiber, as illustrated in FIG. 13, a variation called a wave, a ripple, a hump (bump), or the like (hereinafter, it will be referred to as a “wave” as being a representative thereof) occurs in the wavelength property of the single mode propagation region 20 of the measurement target fiber.
Accordingly, in a case where the reference line 24 is determined by linearly approximating the long wavelength side of the transmission light power ratio A(λ), the cut-off wavelength λ acquired from an intersection 28 of the straight line 26 acquired by shifting the reference line 24 by 0.1 dB and the transmission light power ratio A(λ) on the short wavelength side without avoiding the occurrence of incorrectness, and cannot necessarily be considered as having an accurate value.
In other words, according to the linear approximation technique for a case where the reference line 24 is determined, deviation of different values of cut-off wavelengths from the same measurement data of the ratio of the power of the transmission light cannot be avoided, in other words, a variation occurs in the calculated cut-off wavelength depending on the calculation method.
Here, although the policy of the linear approximation technique for determining the reference line 24 is represented in the standards, a precise processing method for the wave or the like has not been determined.
As a method of measuring the cut-off wavelength that can solve the above-described problems of the multi-mode excitation method, a method called a single mode fiber reference method has been proposed in “2009 Processing 2 of the Society Conference of the Institute of Electronics, Information and Communication Engineers, Page 190, Proposal of Reliable Cut off Wavelength Measurement for Bend Insensitive Fiber”.
According to the single mode fiber reference method, instead of the multi-mode fiber used in the multi-mode excitation method, a single mode fiber having a wavelength shorter than that of the measurement target fiber is prepared as the reference fiber.
Then, the power Psig(λ) of the transmission light transmitted through the measurement target fiber and the power Pref(λ) of the transmission light in a state in which a small bending portion of 60 mmφ is added to a single mode fiber (a fiber having a cut-off wavelength shorter than that of the measurement target fiber) used as the reference fiber are measured.
The ratio A(λ) of the transmission light power Psig(λ) to the transmission light power Pref(λ) is acquired by using the same method as that described above, and a long wavelength-side portion of the spectrum of the transmission light power ratio A(λ) is linearly approximated so as to acquire a reference line.
In addition, a wavelength corresponding to the short wavelength-side intersection of a straight line acquired by shifting the reference line by 0.1 dB and the transmission light power ratio A(λ) is determined as a cut-off wavelength.
In the single mode fiber reference method, a single mode fiber having a cut-off wavelength shorter than that of the measurement target fiber, to which a small bending portion of 60 mmφ is added, is used as the reference fiber.
Accordingly, unlike the case of the multi-mode excitation method, at least a large fluctuation does not occur in the transmission light power ratio A(λ) at least on the side of the long wavelength (single mode propagation region) longer than that at a position close to the cut-off wavelength of the measurement target fiber.
Accordingly, the reference line can be uniquely determined in an easy manner, and, as a result, the cut-off wavelength can be uniquely determined in an easy manner, whereby the occurrence of a variation in the calculated cut-off wavelength can be avoided.
Thus, in the single mode fiber reference method proposed in “2009 Processing 2 of the Society Conference of the Institute of Electronics, Information and Communication Engineers, Page 190, Proposal of Reliable Cut off Wavelength Measurement for Bend Insensitive Fiber”, there are the following problems.
In the single mode fiber reference method, in order to perform accurate measurement, it is necessary to use a fiber having a cut-off wavelength that is sufficiently shorter than that of the measurement target fiber as a single mode fiber of the reference fiber.
In other words, in a case where a single mode fiber having a cut-off wavelength that is slightly shorter than that of the measurement target fiber is used as the reference fiber, between wavelength dependence of the power of transmission light transmitted through the measurement target fiber and that of the transmission light transmitted through the reference fiber, the transmission light power ratio A(λ) that is sufficient for calculating a cut-off wavelength cannot be acquired.
Accordingly, there is a case where it is difficult to accurately calculate a cut-off wavelength.
Accordingly, in the proposal disclosed in “2009 Processing 2 of the Society Conference of the Institute of Electronics, Information and Communication Engineers, Page 190, Proposal of Reliable Cut off Wavelength Measurement for Bend Insensitive Fiber”, such problems are not considered, and, accordingly, there is concerned that accurate measurement cannot be performed.
Furthermore, in a case where the cut-off wavelength of a measurement target fiber is to be newly measured, there are cases where the cut-off wavelength of the measurement target fiber is shorter than the cut-off wavelengths of all the single mode fibers, which are known, prepared in advance.
In such a case, a reference fiber cannot be selected from the single mode fibers prepared in advance.
Consequently, in a practical measurement site, it cannot be determined that an appropriate reference fiber can be easily prepared in the single mode fiber reference method.